Steam generators, particularly those of the coal fired type, can generate harmful emissions. Recent efforts have focused on oxygen firing which, due to the elimination of the inherent nitrogen that occurs with air firing, results in an essentially pure carbon dioxide product gas which can be more efficiently sequestered. Most oxygen fired steam generators utilize significant gas recirculation in order to maintain the required mass flow through the steam generator to support the heat transfer processes. Gas recirculation at high rates adds considerable cost, complexity, and increases the need for auxiliary power.
Pulverized coal steam generators rely on furnace area for controlling heat transfer. These systems inherently also require substantial gas recirculation in order to maintain furnace and convective surface velocities.